====== Second Edition Unix ======

**Release Date:** June 12, 1972\\
**Released By:** Bell Labs Research\\
**Source Code: ** some source code is in the [[https://www.tuhs.org/Archive/Distributions/Research/1972_stuff/|Unix Archive]], browsable in the [[https://www.tuhs.org/cgi-bin/utree.pl?file=V2|Unix Tree]]\\
**Documentation: ** [[https://www.tuhs.org/Archive/Distributions/Research/Dennis_v2/v2man.pdf|2nd Edition man pages]]

The Second Edition of Unix was developed for the PDP-11/20 at Bell Labs by Ken Thompson, Dennis Ritchie and others. It extended the First Edition with more system calls and more commands. This edition also saw the beginning of the C language, which was used to write some of the commands.

The Second Edition, like the first, was only designed to run on the unprotected PDP-11/20 and requires the Extended Arithmetic Element to simulate some operations which would later be supported natively on the PDP-11/45 and other models. This would be the last PDP-11/20 unprotected version of UNIX, as the following Third Edition represents an intermediary PDP-11/45 assembly version of the system and the kernel is rewritten in C before the Fourth Edition.

The code in the Unix Archive is only the source to some of the commands, some of the library functions, and the C compiler. The files in c/ come from the [[https://www.tuhs.org/Archive/Applications/Early_C_Compilers/last1120c.tar.gz|last1120c.tar.gz]] tape, and form a working C compiler for Second Edition Unix.

The files in lib/ come from the libc.sa file which is on the last1120c.tar.gz tape, and form the C library for the compiler.

The files in cmd/ are recreated from the text fragments found on the file [[https://www.tuhs.org/Archive/Distributions/Research/1972_stuff/s1-bits.gz|s1-bits.gz]]. These were reconstructed by Doug Merritt and Warren Toomey, and form the source code to some of the commands in Second Edition Unix.

Another archive, [[https://www.tuhs.org/Archive/Distributions/Research/1972_stuff/s2-bits.tar.gz|s2-bits.tar.gz]], contains what appears to be a dump of the root filesystem of a Second Edition UNIX installation. The contents include binaries of three different formats: those that explicitly call brk upon start, those with a magic number of 405(8), the First Edition a.out(V) format, and those with a magic number of 407(8), the Second Edition a.out(V) format. These format discrepancies may imply the rough linking time of the various objects present.

Changes include:

  * Exit/wait now support returning a status
  * Many system limits are increased (e.g. mounts, buffer sizes)
  * The hog, kill, sleep, and sync syscalls are added

  * Added mount command to the INIT tape
  * Dropped chown, cp, ln, mv, rm, rmdir, stat commands from INIT tape

  * The init system now uses getty and login
  * The shell now supports [ ] character class matches, appending with %%>>%%
  * The shell has expanded programming support with control flow (provided by programs) and error messages
  * Many commands add diagnostics
  * The stty command is added to set terminal characteristics
  * The cp and mv commands no longer operate on pairs
  * The ds, istat, and salv filesystem tools are added
  * NROFF is allegedly added or very close, as it is in the manual %%TOC%% but the page is absent

  * The assembler and linker now support relocation and segmentation
  * The bc B interpreter replaces the old B system
  * The cc C compiler is introduced
  * The m6 macro processor is introduced
  * The tmg compiler-compiler is introduced
  * The fc Fortran compiler replaces the old for compiler
  * The linker now supports the Explor language
  * Libraries move from /etc to /usr/lib

  * The floating point simulator is improved
  * The atan, hypot, and sqrt math functions are added
  * The salloc string library is added
  * The qsort algorithm is added

  * Connectivity to GECOS via the 201 data phone is added
  * IBM 2741 support is added
  * Magtape support is added
  * The motd file is added
  * Removed the binary punched paper tape format and utilities
  * Removed many device-specific commands (e.g. dtf, rkf)

The following papers are mentioned in SEE ALSO sections in the manual:

  * Users' Reference to B - K. Thompson
  * C Reference Manual - D.M. Ritchie
  * M6 Manual - A.D. Hall
  * ROFF Manual - J.F. Ossanna
  * The UNIX Time-sharing System - D.M. Ritchie, K. Thompson

For more information about Second Edition Unix, see [[https://www.bell-labs.com/usr/dmr/www/hist.html|The Evolution of the Unix Time-sharing System]] by Dennis Ritchie.